1. Field of the Invention
This invention relates to lighting systems using light emitting diodes (LEDs) and more particularly to LED based lighting units for illuminating display boxes.
2. Description of the Related Art
Display units, such as light boxes, cabinet signs and box signs are commonly found on the outside of buildings or businesses and are often used to advertise the name of the business or products. Typical units are constructed of aluminum or plastic housing having the shape of a box and are approximately 5″ deep. The housing sometimes has a swing open frame to allow for easily changing the advertising graphics within. The top opening in the housing, or surface, is covered by a translucent or clear lens that transmits light from within the housing. The advertisement graphic is placed under this lens so that it is between the lens and the lighting units inside the light box. This allows the graphic to be illuminated from behind by the lighting units within the light box. In some cases the translucent lens itself may be the illuminated graphic.
Some light boxes or sign cabinets have graphics on one side and light only illuminates that side, whereas others are double-faced such that the two opposite sides of the light box each have a translucent or clear lens with a graphic and lighting inside the light box or sign cabinet illuminates both these sides and graphics.
To enhance the visibility of the advertisement within these units, different types of lighting are incorporated. Various types of lighting systems are used with different light sources such as incandescent bulbs, neon bulbs or fluorescent tubes. One of the problems associated with the conventional lighting units and systems is that their light sources can experience relatively short lifespans and they can have relatively low electrical efficiency. Incandescent bulbs, neon bulbs and fluorescent tubes have a relatively short lifespan, particularly when compared to other light sources, such as typical LEDs. These light sources are also electrically inefficient and providing sufficient lighting, especially in large lighting applications, requires the consumption of significant energy. For example, a standard fluorescent tube 60 inches in length consumes as much as 60 to 70 Watts, and conventional display units can utilize many of these tubes. Neon bulbs can also experience difficulty with cold starting, which can lead to failure of the neon bulb.
More recently, with the advent of the efficient solid state lighting sources, these display units have been used with LEDs, for example. LEDs are solid state devices that convert electric energy to light and generally comprise one or more active regions of semiconductor material interposed between oppositely doped semiconductor layers. When a bias is applied across the doped layers, holes and electrons are injected into the active region where they recombine to generate light. Light is produced in the active region and emitted from surfaces of the LED.
LEDs have certain characteristics that make them desirable for many lighting applications that were previously the realm of incandescent or fluorescent lights. Incandescent lights are very energy-inefficient light sources with a vast majority of the electricity they consume being released as heat rather than light. Fluorescent light bulbs are more energy efficient than incandescent light bulbs, but are still relatively inefficient. LEDs by contrast, can emit the same luminous flux as incandescent and fluorescent lights using a fraction of the energy.
In addition, LEDs can have a significantly longer operational lifetime. Incandescent light bulbs have relatively short lifetimes, with some having a lifetime in the range of about 750-1,000 hours. Fluorescent bulbs can also have lifetimes longer than incandescent bulbs such as in the range of approximately 10,000-20,000 hours, but provide less desirable color reproduction. In comparison, LEDs can have lifetimes between 50,000 and 70,000 hours.
The increased efficiency and extended lifetime of LEDs is attractive to many lighting suppliers and has resulted in LED lights being used in place of conventional lighting in different sign applications. For example, U.S. Pat. No. 5,697,175 to Schwartz, discloses a low power illuminated sign that is particularly adapted for use with common EXIT signs over doorways. The back of each sign comprises a reflector with a series of cavities with curved surfaces. Each cavity corresponds to a letter and background area in the sign. LEDs are mounted in the center of the cavities to illuminate the letters or background area. The LEDs are provided on a separate perpendicular circuit board or on a central projection formed in the bottom of the cavities, with light from the LEDS directed outward. The letters and background area of the sign are illuminated by light reflecting forward from the curved surfaces of the cavities, so that the only visible light is from the illumination of the cavities.
U.S. Pat. No. 6,042,248, to Hannah et al., discloses an LED assembly for channel letter illuminating signs having an enclosure/housing covered by a translucent lens. Each sign includes a plurality of track moldings at the base of its enclosure, with the moldings running along the longitudinal axis of the sections of the channel letter. Linear arrays of LEDs are mounted on printed circuit boards (PCBs) that are then mounted in the track moldings. Each track molding can hold two PCBs in parallel with each of the PCBs arranged on a longitudinal edge, with the LEDs directed outward.
LED based display case or box lighting is also available from Philips Electronics, under the brand name Affinium, which comprises modules containing 3 or 6 LEDs with separate features such as lenses mounted over each of the LEDs. The LEDs and features are enclosed in a housing. The LED modules are mounted inside a display cases to hold the LEDs in place.
In these and similar lighting units generally reflectors or lenses are used over the light emitters to direct their lights. Generally, these lens or reflector features are individually placed over each light emitter making the units difficult and costly to assemble because of the need to place multiple features. Also, these units are more complex and difficult to seal.